1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copying machine or a printer, and more particularly to the toner density control in the developing device thereof.
2. Related Background Art
FIG. 8 is a schematic cross-sectional view of a color image forming apparatus.
This image forming apparatus is provided with a digital color image reader portion in the upper part and a digital color image printer portion in the lower part thereof.
In the reader portion, an original 30 is placed on an original supporting glass plate 31 and is scanned by an exposure lamp 32, and the light reflected from the original 30 is focused by a lens 33 onto a full-color sensor 34 to obtain color separated image signals, which are supplied through an unrepresented amplifying circuit, processed in a video process unit and supplied to the printer portion.
In the printer portion, a photosensitive drum 1 constituting an image bearing member is supported rotatably in a direction R1, and, around the photosensitive drum 1, there are provided a pre-exposure lamp 11, a corona charging device 2, an exposure optical system 3, a potential sensor 12, four developing units 4Y (yellow), 4C (cyan), 4M (magenta) and 4K (black), an on-drum toner detecting means (sensor) 13, a transfer unit 5 and a cleaning device 6. The laser beam exposure optical system 3 receives the image signals from the reader portion and converts the signals into an optical signal by a laser output portion (not shown). The laser beam is reflected by a polygon mirror 3a, then guide by lens 3b and mirror 3c and is converted into an optical image E which linearly scans (raster scan) the surface of the photosensitive drum 1.
At the image formation in the printer portion, the photosensitive drum 1 is rotated in the direction R1, at first subjected to charge elimination by the pre-exposure lamp 11, then uniformly charged by the charging device 2 and is irradiated with the optical image E of each separated color to form a latent image. Then, for each separated color, a predetermined developing unit is activated to develop the latent image on the photosensitive drum 1, thereby forming an image with toner, principally composed of a resinous material, on the photosensitive drum 1. In this operation, the latent image is developed with the developer, utilizing an electric field formed in the developing area.
The developing units 4Y, 4C, 4M, 4K are so constructed that one unit corresponding to the separated color selectively approaches to the photosensitive drum 1 by the function of eccentric cams 24Y, 24C, 24M, 24K. The toner image on the photosensitive drum 1 is transferred onto a recording material, which is supplied from a cassette 7 (7a, 7b or 7c) through a conveyor system and the transfer device 5 to a position opposed to the photosensitive drum 1.
The transfer device 5 is provided in this example with a transfer drum 5a, a transfer charger 5b, an adsorption roller 5g opposed to an adsorption charger 5c for electrostatically absorbing (attracting) the recording material, an internal charger 5d and an external charger 5e. On the peripheral aperture of the rotatably supported transfer drum 5a, there is integrally adhered, in cylindrical shape, a recording material supporting sheet 5f consisting of a dielectric material. The recording material supporting sheet 5f is composed of a dielectric sheet such as a polycarbonate film.
With the rotation of the transfer drum 5a, the toner image on the photosensitive drum 1 is transferred, by the function of the transfer charger 5b, onto the recording material supported on the recording material supporting sheet 5f. In this manner the toner images of a desired number of colors are transferred onto the recording material, transported by the recording material supporting sheet 5f by attraction thereto, thereby forming a multi-color image.
In case of a four-color mode, after the transfer of the toner images of four colors, the recording material is separated from the transfer drum 5a by the function of a separating finger 8a, a separating push-up roller 8b and a separation charger 5h, then subjected to image fixation in a heat roller fixing device 9 and discharged to a tray 10. On the other hand, the photosensitive drum 1 after the image transfer is subjected to the cleaning of the surfacially remaining toner by the cleaning device 6 and is used again for image formation.
In case images are formed on both faces of the recording material, a transport path switching guide 19 is activated after the recording material is discharged from the fixing device 9 to introduce the recording material into an reversing path 21a through a vertical path 20. After the recording material is once stopped, an reversing roller 21b is reversed to advance the recording material in the opposite direction, with the trailing end of the sheet at the introducing operation as the leading end, whereby the recording material is stocked in an inverted state in an intermediate tray 22. Thereafter an image is formed on the other face through the image forming process explained above.
The recording material supporting sheet 5f of the transfer drum 5a is smeared by the toner scattered from the photosensitive drum 1, the developing units 4Y to 4K, the cleaning device 6 etc., toner deposition at the jamming of the recording material, the deposition of oil from the recording material at the image formation etc. It is cleaned however by a fur brush 14, a backup brush 15 opposed thereto across the recording material supporting sheet 5f, an oil removing roller 16 and a backup brush 17 opposed thereto across the recording material supporting sheet 5f, before it is subjected again to the image forming process. Such cleaning is executed at the pre-rotation step and the post-rotation step, and also when the sheet jamming occurs.
In the present example, the gap between the recording material supporting sheet 5f and the photosensitive drum 1 can be set at a predetermined value at a predetermined timing, by activating a transfer drum eccentric cam 25 to drive a cam follower 5i integral with the transfer drum 5a. It is thus possible, for example in a stand-by state or when the power supply is turned off, to separate the transfer drum 5a from the photosensitive drum 1, thereby separating the rotation of the transfer drum from the rotary drive of the photosensitive drum.
In the image forming operations described above, the developing units 4Y to 4K and a developer density control device function in the following manner. In FIG. 9, a developing unit 4 stands for any of the developing units 4Y to 4K, and there are shown developer agitating screws 42, 43 provided in the developing unit 4.
After the reading of the original 30 shown in FIG. 8 by a CCD and the formation of the electrostatic latent image on the photosensitive drum through the image forming steps, when such latent image reaches the developing position, a developing bias voltage, consisting of superposed AC and DC voltages, is applied by a developing bias source 40 to a developing sleeve 41 of the developing unit 4 shown in FIG. 4. At the same time, the developing sleeve 41 is rotated in a direction C by an unrepresented developing sleeve drive device and is pressed by a developing pressure cam 24 (24Y to 24K) so as to be positioned at a predetermined distance from the photosensitive drum 1, and the latent image is developed in such state.
At the same time, the input image signal read by the aforementioned CCD is subjected to A/D conversion, and the density level in each pixel is counted by a video counter (not shown). According to the accumulated value of the count, a CPU (not shown) determines the toner replenishment amount, and the toner is thus replenished from a hopper 44 to the developing device 4.
There may be employed, in addition, a developer density control device which detects the variation in the image density by the output voltage of a sensor. The image forming apparatus shown in FIG. 8 is provided with such control device, which forms the latent image of a predetermined patch image for density detection on the photosensitive drum 1, developing such latent image in the above-described manner to form a patch image, reading the density thereof with a sensor 13 and accordingly replenishing the toner from the hopper 44 to the developing device 4. Thus the developer density is so controlled that the patch image always has a constant density.
During the image forming process, the patch image is normally formed in a non-image area, and the developer density control utilizing such patch image is executed once in every tens of image forming cycles.
However, the toner replenishment amount to the developing device 4 (4Y to 4K) inevitably fluctuates by the toner refilling to the toner hopper 44 of the developing device 4 or by the individual fluctuation of the toner hopper 44 itself, whereby the toner replenishment of the desired amount cannot be achieved in stable manner, eventually leading to instability in the image density.